Pressure relief bypass valves are often used in fluid delivery systems to establish a bypass path in the event of an overpressure condition. A typical bypass valve includes a spring-loaded piston seated on an internal orifice of a pump or other system member. The spring biases the piston into a sealed or closed position, and when the system pressure reaches a level sufficient to overcome the preset bias of the spring—sometimes referred to as the differential pressure set point—the piston is lifted from the orifice to allow fluid flow there through.
While effectively compensating for liquid overpressure conditions, bypass valves have not historically aided in the resolution of underpressure conditions, such as when a delivered fluid transitions to a vapor phase and/or entrained air condition. A fluid delivery system of interest is one that is carried out by connecting a hose from a tank of a delivery vehicle to a customer tank. The fluid is typically a liquid/vapor fluid that is pumped and metered from the vehicle to a customer's tank.
In the case of a vaporous fluid, such as hydrocarbon fuels like fuel oil and diesel, government regulations consider the delivery vehicle to constitute the point of sale and prohibit the sale of vapor and/or air, such as can occur should the delivery tank be emptied of liquid and the pump continue operating. Such systems often utilize a positive displacement pump that continues to deliver vapor and/or air after the delivery tank has been emptied of liquid.
Meters in the past were often provided with vapor eliminator stages to prevent the metering of vapor, but such meters have not always been effective as they can be overloaded and do nothing to prevent or alleviate the pumping of the fluid when a transition occurs to the vapor state.
There is therefore a continuing need for a bypass valve that effectively compensates for both overpressure and underpressure conditions, and which accommodates vapor phase conditions in fluid delivery systems. It is to such an improvement that the present invention is generally directed.